US3452396A - Die center pad - Google Patents

Description

y 1, 1969 E. B. GARVER ETAL 3,4

DIE CENTER PAD Filed July 14, 1967 I Ll Iuvsm'oas Euwmao B. GARVER RANDOLPH D. Lume w cw.

y 1, 1969 E. B. GARVER ETAL 3,

DIE CENTER PAD Filed July 14, 1967 Sheet 2 of 3 haven-mas EDWARD 5.6ARVER RANDOLPH D. Lums f Arm y 1969 E. B. GARVER ETAL 3,452,396v

DIE CENTER PAD Sheet Filed July 14, 1967 an" v. 3% .7. NA A 56 v W ow M m hi & J .L

United States Patent 3,452,396 DIE CENTER PAD Edward B. Garver, Chicago, Ill., and Randolph D. Lurie, Parkersburg, W. Va., assignors to Continental Can Company, Inc., New York, N.Y., a corporation of New York Filed July 14, 1967, Ser. No. 653,416 Int. Cl. B29c 17/00 U.S. Cl. 1819 6 Claims ABSTRACT OF THE DISCLOSURE A central male cup-forming die in a cold drawing process has an exterior wall conditioned for frictionally engaging the inner wall of a cup being formed as it is drawn between a draw ring into a space about said central male cup-forming die, the frictional engagement generatinga force overcoming resistance of the cup-forming fabric to flow into said space to minimize strain in the forming cup as said central male cup-forming die and draw punch and draw ring move relatively.

This invention relates to a die assembly for forming plastic containers. Particularly it relates to a die center pad which is adapted to preclude thinning of plastic material adjacent bottom walls of cold drawn plastic containers. Such phenomenon is experienced when using heretofore known assemblies and methods.

Independent of the final shape or contour of a redrawn plastic container or body, a conventional deep-drawing process, which includes a redrawing operation, may generally consist of three basic operations.

The first operation is performed by a first die assembly. It combines functions of cutting a blank from suitable plastic material, such as polyethylene, to proper size and drawing such blank into a relatively shallow container body. During the first operation, a fiat plastic sheet is clamped between a female die or draw punch and a draw ring of the first die assembly. Simultaneously the plastic sheet is trimmed into a blank by the cutting action of the draw punch which forces peripheral parts of the plastic fabric of the sheet past a shear fitting closely about the draw punch.

In the first operation, the portion of the blank which forms the container side wvall is drawn from the fabric which is clamped between the draw punch and the draw ring. Downward movement of the draw punch and the draw ring causes the side wall forming fabric to be drawn into a narrow space between the draw punch and a conventional male die or center pad of the first die assembly. At the completion of the first operation, the container body comprises a shallow container side wall joined by a radius portion to a bottom wall. It may also include a radially outwardly directed flange formed from peripheral portions of the blank.

The second operation converts the shallow drawn container body into a container body of greater depth and smaller diameter by redrawing with a second die assembly. Prior to the second operation, the shallow drawn container is arranged in the second die assembly in a manner such that the side wall of said container extends downwardly over the outside surface of a draw ring, and the inside surface of said container bottom wall supportingly engages the upper end surfaces of said last draw ring and a male die or center pad which comprise said second die assembly.

As the second operation begins, a central area of the container bottom wall is clamped between a knock-out pad and the end surface of the male die. Immediately following this clamping action, a female die or draw punch of the second die assembly contacts a fabric ring beyond the knock-out pad. The fabric ring then defines the outer area of the container bottom wall. The force of the contact by the draw punch produces a slight clamping pressure between the draw punch and the draw ring.

During the second operation, said fabric ring is incorporated in the side wall of the redrawn container by reason of downward movement of the drawn punch. The latter member pushes the draw ring in a manner such that the material of the fabric ring is drawn into a space between the draw punch and the male die. As the material of the original fabric ring is drawn from the space between the draw punch and the draw ring, it is replaced by new material drawn into said last space from outside the draw ring. The new material is incorporated into the side wall of the redrawn container as the second operation conventionally continues until all of the fabric outside of said draw ring has been drawn into said last space.

At the completion of the redrawing or second operation, not all of the fabric may have been drawn into the space between the draw punch and the male die. Such remainder forms a radial outwardly extending flange of the redrawn container.

The third operation may comprise two steps. First, the flange of said redrawn container is contoured for closure reception. Second, said flange is trimmed for compatibility with a closure. The instant invention is not concerned with the third operation. Accordingly, further consideration thereof is omitted.

During the described first and second operations, and most likely during the second thereof, the redrawn container tends to thin adjacent the juncture of its side wall and its bottom wall. As a result, several things may occur. First, the fabric of a container may strain sufficiently to cause a brittle break or tear. Second, thinned fabric may propagate along a container side wall and/ or bottom wall and thus render a container commercially useless. Moreever, the fabric of a container may strain harden upon application of tensile forces too weak to cause tearing in a manner such that a thin but relatively strong and highly oriented band may be formed around a container body.

The problem of thinning may be solved in various ways as, for example, by increasing the number of redraws before container completion. However, the increased number of redraws proportionally increases cost of container manufacture. The problem of thinning may also be solved, as in accordance with the present disclosure, by providing means in a die assembly for controlling elongation of a fabric to a level within its elastic limits during drawing or forming operation.

Leading to the present invention was recognition for the first time that elongation of fabric beyond its elastic limit during a first drawing operation or redrawing operation results from generation or application by a draw punch, along an adjoining face of fabric for a container being formed, of strain forces of a magnitude sufiicient to impede fabric flow. In accordance with the present invention and as a feature thereof, a die assembly is constructed in a manner such that the effect of such strain forces is minimized. In accordance with the particular embodiments of the invention herein illustrated, such strain forces are overcome by opposing forces applied along a fabric surface which has become the inner surface of a container being formed. The negating forces of strain effect of said particular embodiments result from arrangement, conditioning and/or proportioning of a male die or center pad in a manner such that frictional and other holding forces are generated between the outer surface of a male die or center pad and a thereto proximate fabric surface which resist strain forces applied by a draw punch.

The foregoing and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description and appended claims, when considered in conjunction with the accompanying drawings wherein the same reference character or numeral refers to like or corresponding parts throughout the several views.

On the drawings:

FIGURE 1 is a schematic sectional view of a die assembly embodying one form of the present invention conditioned with a first drawn container immediately prior to a redrawing operation, parts shown in phantom for the purpose of illustration.

FIGURE 2. is a view similar to FIGURE 1, said die assembly, however, conditioned in an intermediate stage of a redrawing operation.

FIGURE 3 is a view similar to FIGURE 1, said die assembly conditioned at completion of a redrawing operation.

FIGURE 4 is an enlarged sectional view of a fragment of said die assembly conditioned shortly after commencement of the redrawing operation.

FIGURE 5 is an enlarged veiw similar to FIGURE 4, however, with said assembly intermediately conditioned as ab out in FIGURE 2.

FIGURE 6 is a further, enlarged sectional view of a fragment of an upper portion of said die assembly when conditioned as in FIGURE 4, the relationship of illustrated objects being exaggerated for the purpose of illustration.

FIGURE 7 is a sectional view on the scale of FIG- URE 6 and illustrating in exaggerated form the condition of a fragment of said die assembly immediately below the fragment of FIGURE 6, and arranged at a stage between the stages shown in FIGURES 4 and 5.

FIGURE 8 is a sectional view on the scale of FIG- URB 6, however, illustrating in exaggerated form the condition of a fragment of said die assembly comprising the lower part of FIGURE 7, and conditioned as in FIGURE 5.

FIGURE 9 is a enlarged elevational view of a fragment of a male die or center pad comprising a modified form of the invention with circumferential grooves disproportionately presented for the purpose of said invention.

FIGURE 10 is an enlarged elevation of a male die or center pad comprising yet another modified form of said invention.

Referring first to FIGURES 1-3, inclusive, in connection with which a redrawing process will be described generally, a die assembly generally designated A comprises a lower die body L and an upper die body U. Said lower and upper die bodies, respectively, are reciprocatively arranged in axial alignment in a conventional press (not shown) in a manner heretofore well known in the art and forming no part of the present invention. For that purpose, lower die body L is secured by conventional means to a bolster plate B. Said bolster plate is in turn securely fastened to the unshown press.

Said lower die body comprises a base plate 13. A plurality (preferably four) of identical vertical guide pins 14 are symmetrically arranged and project upwardly from base plate 13 to which they are secured. Said upper die body comprises a reciprocative plate 16. Said plate has a plurality of vertical bores or openings 15 which are arranged for sliding reception of guide pins 14. Each of bores 15 may be provided with a conventional anti-friction bushing (not shown) to facilitate relative reciprocative sliding movement of plate 16 longitudinally of the guide pins 14.

Plate 16 has an upwardly projecting axial stem 17. Said plate and stem have an axial bore 18. A conventional vertically reciprocative ram R of the press (not shown) is disposed intimately about said stem with a lower flat of said ram rigidly secured to an upper flat of plate 16 by any suitable means such as bolts (not shown).

Plate 16 has an annular downwardly opening recess 21.

4 An upper end portion of a generally cylindrical downwardly projecting female die or tubular draw punch 20 is secured by conventional means in recess 21. Draw punch comprises an inner wall 22, an outer or circumferential wall 24, and a lower annular end wall or face 25. Wall 22 defines a guide bore or chamber 23.

A knock-out pad 27 comprises a knock-out head 26 with a lower disc face 30 and an upwardly projecting stem 28. Knock-out head 26 is arranged for vertical reciprocation in guide bore 23, and stem 28 is slidably received in axial bore 18. Stem 28 is biased by yieldable spring means (not shown) to maintain lower end face 30 flush with annular face 25 when the parts are conditioned as in FIGURE 1.

Base plate 13 defines an upwardly opening recess 32. A generally cylindrical retainer 31 is secured in said last recesses by means (not shown) which may be conventional. Retainer 31 comprises an inner circumferential wall 33 which defines a guide bore or chamber 34. Wall 33 terminates at an inwardly directed annular flange 35 which forms a stop at the top of said last chamber. An upwardly biased draw ring, generally designated 36, is arranged for vertical reciprocation in chamber 34. Flange 35 is adapted to limit upward movement of said draw ring.

Draw ring 36 is disposed in axial alignment with draw punch 20, and said draw ring has a vertical axial bore 37 which is deposed co-axially with the guide bore 23. Said draw ring also defines an end face 38 which opposes the end face 25 of draw punch 20 and has an exterior circumferential wall 40. Wall 40 is contoured to mate with an internal surface of wall 39 of an inverted plastic first drawn container C as illustrated in FIGURE 1. Draw ring 36 has an annular flange 41 proportioned for abutment with flange 35 to limit upward movement of said draw ring as aforesaid and as illustrated in FIGURE 1. A plurality (preferably four) of symmetrically arranged identical draw ring guide pins 42 are rigidly secured by conventional means to and depend from flange 41. Base plate 13 has a plurality of identical vertical bores 43 adapted for vertically relative reciprocation of guide pins 42. Each of said last guide pins is secured to a conventional air cushion pressure pad 44. Pressure pad 44 and therefore said draw ring are biased upwardly to the position illustrated in FIGURE 1, preferably by conventional pneumatic or hydraulic means (not shown).

A male die or center pad 45 is rigidly secured to the base plate 13 from which said male die projects upwardly in axial alignment with the guide bore 23. Male die 45 defines a generally cylindrical body 46 terminating at its upper end in an end face 47 of substantially circular outline. An opposite end portion of said male die has an integral radial mounting flange 48. Base plate 13 has an upwardly opening recess 50 in which flange 48 is secured. Suitable fasteners such as a plurality of bolts 51 with countersunk heads, only one of which is shown, may be provided for said securance.

Operation of die assembly A will be best understood by first referring to FIGURE 1 in which conditioning just after beginning a redrawing cycle of the conventional press (not shown) is illustrated. Just prior thereto, ram R is disposed above the position shown in FIGURE 1 to permit spacing of annular face 25 from annular face 38 a distance adequate to enable mounting of first drawn container C in an inverted position as shown in FIGURE I. Said first drawn container is of plastic fabrication, and, in the herein illustrated embodiments prior to the condition of FIGURE 1, has been formed in a first operation not illustrated in the drawings but heretofore described.

From its position just prior to FIGURE 1, ram R is reciprocated downwardly until end face 30 and annular face 25 engage the bottom wall 29 of said container. Such condition is between the conditions shown in FIGURES 1 and 2. Upon continued downward movement of said ram, a central circular area (FIGURES 4 and 5) of container bottom wall 29 is clamped between the end face 30 of knock-out head 26 and end face 47 of male die 45. Area 60 will become the bottom wall of the redrawn container. The parts are arranged and reciprocated so that substantially simultaneously with such clamping action annular face 25 exerts a pressure which clamps the remaining part 61 of bottom wall 29 against end face 38 of upwardly biased draw ring 36. It is observed that remaining part 61 is disposed beyond knock-out pad 26. At this stage of the redrawing operation, there is no clamping action on the container wall or body of said container.

A later stage of the redrawing operation is illustrated in FIGURE 2. In such condition, the central area 60 of bottom wall 29 is still firmly clamped between knock-out pad 27 and end face 47 of said die center pad. The material which is to form the wall or body of the redrawn container is supplied from a fabric ring clamped between the opposed annular end faces 25 and 38. This fabric ring continually changes throughout the redrawing operation. Initially, such ring consists only of material which is outer part 61 of the bottom wall 29 of the original drawn container. However, as draw punch 20 moves downwardly against the bias of said draw ring to the stage of FIGURE 2 and thereafter, material theretofore clamped between faces 25 and 38 has been incorporated into the container side wall by drawing into a cylindrical space 62 between the outer wall 53 of said male die and the inner wall 22 of said draw punch. The parts are proportioned to so that inner wall 22 tends to urge fabric drawn from between the faces 25 and 38 into intimate engagement with the exterior peripheral surface 53 of the male die 45 to thereby form the side wall of the redrawn container.

In FIGURE 3, the redrawing operation has been completed. Upper die body U is ready to return to its upper (rest) position. The extreme downward position of ram R and draw punch 20 determines the depth of the finished container. Not all of the plastic fabric has been drawn into the redrawn container wall. A radial flange 54 (FIG- URE 3) remains at the end of the redrawn container opposite the bottom wall 29. Flange 54 is adapted for formation for lid securance in a manner forming no part of the present invention. Accordingly, further description is omitted.

Attention is now invited to FIGURES 4-8, inclusive, in connection with which improvement in accordance with the present invention will be identified in the ensuing description. At the outset, however, it is observed that surfaces 22, 25 and 38 are exceedingly smooth, preferably highly polished, to minimize the coefficients of starting and sliding friction for the relatively movable parts. Thereby, resistance to flow of fabric from between surfaces 25 and 38 and into cylindrical space 62 is minimized.

Notwithstanding such surface conditioning, the magnitude of impact of draw punch 20 and fabric part 61 upon commencement of a redraw operation, when using conventional materials, is such that the frictional force generated overcomes the tendency of the fabric to flow thereby stretching the affected fabric. However, the parts are conventionally propontioned and the character of conventional fabric is such that before tension in the strained fabric reaches tensile strength, the aforesaid frictional forces will be overcome and an initial flow of fabric between surfaces 25 and 38 commences. To graphically illustrate the consequences of such phenomena, it is observed that cylindrical space 62 prior to engagement of draw punch 20 and container C may be considered as defined by inner wall 37 of draw ring 36. The latter wall may be formed on the same radius as that of wall 22. The upper end of space 62 is an annular gap 64 bridged by a narrow fabric ring 63. Ring 63 as well as container center part 60 will be strained as a consequence of initial impact on the fabric (which resists tendency to flow) as said draw punch moves from the condition of FIGURE 1 to the condition of FIGURE 4, as illustrated in FIGURES 4 and 6. Accordingly, as fabric thins there is insufficient fabric bulk to fill the upper end of space 62. However, the conventional character of the fabric is such that before the elastic limit of fabric ring 63 is reached, the material will begin to flow from between surfaces 25 and 38 and, as intended, extend across space 62 in engagement with die surface 53 and inner wall 22, as illustrated in FIGURE 7.

In the meantime, another counter-flow force develops because of circumferential compression of the fabric as said draw punch moves downwardly from the position of FIGURE 1 to the position of FIGURE 4. The accumulated mass 61 (FIGURE 7) enters the space between surfaces 25 and 38 once initial fabric flow has been initiated. However, because the parts conventionally are proportioned for a uniform fabric flow, space 62 cannot accommodate such mass when it is in position for entry. As a consequence, mass 61 generates pressure against surfaces 25 and 38 of a magnitude sufficient to again stop fabric flow in a conventional die assembly. During ensuing downward movement of said draw punch in a conventional assembly, fabric will again be strained and likely may harden or perhaps tear. However, should the then built tension overcome the last flow stoppage before the elastic limit is reached, the fabric will again flow, but that condition likely would be succeeded by another flow stoppage for the same reason that caused the prior flow stoppage, and so on.

In accordance with the present invention, to negate the effect of the forces resisting flow of fabric between surfaces 25 and 38 after flow has once been initiated and thereby preclude hardening or rupture of the fabric, 'a gripping force is applied to the inner surface 66 (FIG- URES 5 and 8) of said redrawn container at a level which is spaced from surface 47. The level at which flow initiates determines such last spacing. That is to say, the fabric in space 62 should be of suflicient bulk to be gripped by gripping means to be hereinafter defined. Such sufficient bulk will be available once initial thinning has ceased.

In accordance with the embodiment of the invention illustrated in FIGURES 1-8, inclusive, said gripping means comprises a circumferential elevation or collar 67 which is fashioned integrally with outer wall 53 of said male die. Said collar extends into and constricts space 62 at a level which is disposed adjacent the position at which fabric flow commences from between surfaces 25 and 38. Said collar generates a force upon engagement with the fabric of the container being redrawn which is adequate to sustain normal flow between surfaces 25 and 38 by overcoming the flow counter-forces of fabric mass 65. If collar 67 is too close to surface 47, it cannot produce the desired result because of lack of fabric to grip. However, if said collar is suitably disposed, fabric flow, once begun, will be sustained throughout the remainder of the redraw operation.

After completion of the redraw, the formed container shrinks normally longitudinally. Accompanying fabric flow thickens, to the dimension of the remainder of the Wall of redrawn container, the theretofore thinned fabric ring 63.

The location and dimensions of collar 67 will vary according to the fabric of a container, its thickness, the transverse dimension of space 62, and fabric flow characteristics. Presently, determinations for optimum design of a male die embodying the present invention are empirical.

A particular example of successful redraw procedure in accordance with the present invention forms a polycarbonate container having a side wall thickness of about .014". A collar 67 for this purpose extends radially from wall surface 53 into space 62 a distance of .002". The height of said collar is .170" and it has opposite blending surfaces, each of which is .008" in length, tapering from said collar to said last side wall. Moreover, the upper limit of said collar is spaced about .335" from top surface 47 of said male die.

Other gripping means for developing frictional or gripping force on the interior of the redrawn container to overcome the aforesaid flow resisting forces are considered equivalent to the embodiment of FIGURES 18, inclusive. Modified gripping means are illustrated in FIG- URE 9 which discloses that in lieu of collar 67, a band 77 is provided. Said band is spaced from top wall 47 of said male die for reasons heretofore assigned in connection with spacing of collar 67, and is of equal height thereto. However, said band does not constrict space 62 but comprises a plurality of circumferential grooves defined in wall 53. Said grooves in a working embodiment of the invention are triangular in cross section are .002" wide and .001 deep. They are spaced on .010" centers. The male die of FIGURE 9 is effective in redrawing containers of a side wall thickness of .014" and fabricated from sheet stock extruded from acrylonitrile-butadienestyrene terpolymer. Satisfactory polycarbonate containers also may be redrawn. Similarly fabricated containers redrawn with dies lacking an improvement according to the present invention ruptured or tore for reasons heretofore indicated.

In a further modification of the invention shown in FIGURE 10, surface 53 of the male die is conditioned by sandblasting for gripping the interior surface of a container being redrawn. An effective male die in accordance with the last modification is fabricated from cold rolled steel and sandblasted with random sized grit using 70 to 90 p.s.i. air pressure to roughen surface 53, as at 78. While to produce a desired flow effect it is only required to condition said male die by sandblasting in an area corresponding to the areas of collar 67 and band 77 of the previously described forms of the invention, in the present embodiment the entire surface is sandblasted because it is more economical than trying to limit the area of roughening. The conditioning of additional areas of said male die does not undesirably affect the redrawing process.

A male die conditioned by sandblasting in accordance with the last modification eliminates thinning and tearing or rupture of the character heretofore described in containers fabricated from polycarbonates from various sources and .015" thick, acrylonitrile-butadiene-styrene terpolymer .014" and .0125" thick, and polyvinylchlorides of multiple formulations and being .012" thick.

As many substitutions or changes could be made in the above described construction, and as many apparently widely different embodiments of the invention within the scope of the claims could be constructed without departing from the scope and spirit thereof, it is intended that all matter contained in the accompanying specification shall be interpreted as being illustrative and not in a limiting sense.

We claim:

1. In apparatus for cold drawing a plastic first member, through a stage in which there is formed a transition second member of changing form, into a cup-like third member, said apparatus including a center pad male die having an elongated body, female means including a female die having an inner smooth wall defining a cavity for telescopic reception of said body during a drawing operation, means for normal positioning said first member between said male die and said female die prior to relative movement thereof and means for moving said male die and said female die relative to each other whereby a portion of said first member is drawn across said female means from said normal position to a second position about said body between an exterior surface of said male die and said inner wall during formation of said third member, the improvement comprising means on a fabric engaging surface of one of said dies for generating a gripping force between the outer surface of said body and said transition second member sufficient to overcome flow stopping forces impinged on said transition second member to thereby cause flow of plastic from the first member about said body during relative movement of said male and female dies.

2. Apparatus as defined in claim 1 in which said means for generating a gripping force is arranged on said body for frictionally engaging a thereto adjacent surface of said second member with a force sufficient to overcome the force of said female means tending to strain said second member.

3. Apparatus as defined in claim 1 in which said body comprises an end at which the cold drawing begins, said means for generating a gripping force comprising a circumferential band fashioned on the outer surface of said body and spaced from said end at a level where drawn fabric of the second member is of sutficient bulk simultaneously to engage the outer surface of said male die and said inner wall.

4. Apparatus as defined in claim 3 in which said band comprises a plurality of parallel circumferential grooves fashioned in said body.

5. Apparatus as defined in claim 3 in which said band projects from the exterior of said body toward said inner wall.

6. Apparatus as defined in claim 2 in which said means comprises a randomly pitted exterior body surface.

References Cited UNITED STATES PATENTS 2,270,187 1/ 1942 Dulmage. 2,547,331 4/1951 Lent. 2,854,694 11/1958 Mumford. 3,235,639 2/1966 Knowles. 3,305,158 2/1967 Whiteford.

WILLIAM J. STEPHENSON, Primary Examiner.

US. Cl. X.R. 72--347

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